We investigate the electroluminescence spectra of a series of double- and triple-layered devices based on a rare-earth complex PTT,tris-(1-phenyl-3-methyl-4-isobutyryl-5-pyrozolone)-bis(triphenyl phosphine oxide) terbium. In triple-layer devices, besides green electroluminescence from the light-emitting layer PTT, blue emission from the hole transport layer TPD (N,N'-bis (3-methylphenyl)-N,N'-diphenyl-benzidine) occurs at high electric field strength (EFS) and its peak intensity increases with EFS, This indicates a charge carrier tunneling injection mechanism. At high EFS, the Fowler-Nordheim (FN) plot is close to Linear, supporting the tunneling mechanism. The large barriers at the cathode and anode interfaces are responsible for this FN behavior. In double-layer devices where the 8-hydroxyquinoline-aluminum layer is absent, TPD emission dominates the electroluminescence, proving that PTT possesses high electron mobility. (C) 1998 Elsevier Science B.V. All rights reserved.